BBC NEWS, Friday, 22 November, 2002, 00:00 GMT
Cannabis link to depression
Frequent cannabis use can trigger depression, a study suggests. The findings applied to heavy cannabis users
Researchers have also found further evidence the drug can significantly increase the risk of schizophrenia.
The risks are outlined in three papers in the latest issue of the British Medical Journal.
Researchers say their findings highlight the need for measures to reduce frequent and heavy use of cannabis.
The first paper, by doctors in Australia, found frequent cannabis use among teenage girls in particular can trigger depression.
Their seven-year study of 1,600 teenage girls found girls who used the drug everyday were five times more likely to become depressed and suffer from anxiety compared to those who did not use the drug.
Those who used the drug at least once every week were twice as likely to develop depression compared to non-users.
A second study, by doctors in Sweden, confirmed previous research suggesting that cannabis can increase the risk of developing depression.
Their study of more than 50,000 men found those who had smoked the drug in the late 1960s were 30% more likely to have developed schizophrenia.
The authors said their results suggested that as many as one in eight cases of schizophrenia in the UK could be prevented by stopping people from using cannabis.
The third study, by British scientists, suggests the risks of developing schizophrenia are highest for those people who use the drug when they are a teenager.
Their study of more than 1,000 people in their early twenties in New Zealand suggested that one in 10 people who used cannabis as a teenager have since been diagnosed with schizophrenia.
All of the researchers said the ill-effects were linked to cannabis and not to any other drugs.
They also said there was little evidence to suggest that occasional use of cannabis had a similar effect.
In an accompanying editorial Joseph Rey, professor of child and adolescent psychiatry at the University of Sydney, said the studies backed up previous research.
"These findings strengthen the argument that use of cannabis increases the risk of schizophrenia and depression."
But he added that further research is needed.
"Whether the use of cannabis triggers the onset of schizophrenia or depression in otherwise vulnerable people or whether it actually causes these conditions in non-predisposed people is not yet resolved."
The UK charity Rethink, formerly the National Schizophrenia Fellowship, said the findings highlighted the need for a public education campaign on the risks of using cannabis.
Cliff Prior, its chief executive, said: "The research highlights how cannabis can be one of the triggers for severe mental illnesses like schizophrenia and depression.
"It shows why it is so important that more work of this kind is done so that people with severe mental illness can have the best chance of recovering a meaningful quality of life."
He added: "Cannabis is not a risk-free drug. The public needs to understand the potential dangers of triggering mental illness."
Marjorie Wallace, chief executive of mental health charity SANE, added: "The most worrying revelation of these studies is not just the immediate triggering of hallucinations but that cannabis can lead to psychotic symptoms and depression in later life."
ABC NEWS ONLINE, Sun, Nov 24 2002 5:22 AM AEDT
Cannabis linked to depression and schizophrenia
Frequent use of cannabis boosts the risk of depression and schizophrenia, according to studies published in the British Medical Journal (BMJ).
A study in Victoria followed 1,601 school students, aged 14-15, for seven years, regularly checking their health and interviewing them about their lifestyle.
The study showed that 60 per cent of the students had used cannabis by the age of 20 and 7 per cent were daily users.
The research also showed that among frequent users, young women were especially prone to reporting depression and anxiety.
Those who used it daily were more than five times likelier to have these symptoms, compared with non-users, whereas weekly users had a two-fold risk.
"These findings contribute to evidence that frequent cannabis use may have a deleterious effect on mental health beyond a risk for psychotic symptoms," say the authors, led by George Patton, a professor in adolescent health at the
Murdoch Children's Research Institute in Parkville, Victoria.
Some opinion suggests that the picture may be the other way round, and that people who are depressed smoke cannabis in order to make themselves feel happier.
However, Patton's team found that teenagers who were depressed or anxious were not more likely to go on to smoke cannabis.
The report speculates that because the depression risk appears to be confined mainly to daily users, the answer lies in "a direct pharmacological effect".
Cannabis, they argue, affects receptors in specific brain cells that influence memory, emotions, cognition and movement.
They also argue a "counter-cultural lifestyle" and other "psychosocial mechanisms" could compound the affects of cannabis use.
A Swedish study, also published in the BMJ, found a dramatic incidence of schizophrenia among heavy users of cannabis.
The study found those who had smoked it more than 50 times were at least six times likelier to be diagnosed with schizophrenia.
The research is based on more than 50,000 subjects whose health and well-being were at first monitored in 1969-70, and which were routinely followed up until 1996.
The volunteers self-reported their use of cannabis and other drugs and any mental disorder was formally diagnosed by psychiatrists.
The findings show a consistent causal relationship between schizophrenia and intensive cannabis use.
It states the "association is not explained by sociability personality traits, or by use of amphetamines or other drugs."